Search found 69 matches

by terkio
Tue Feb 19, 2019 6:55 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

A trim would take care of gross transistor mismatch. I agree, it would be nice to have no trimmer at all if this can be done without tight transistor matching. Here this depends of the user. Hobbyists who have time to match pairs from hundreds of transistors. Companies that has no time to do that. R...
by terkio
Tue Feb 19, 2019 5:36 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

1) Indeed the differential Deboo is a very interesting circuit. I saw a Deboo integrator needs resistor matching. Some R1*R2/R3 that makes a negative resistor that must equal a fourth resistor R4. Does that need accurate resistor matching ? What about stability ? Negative resistors are a way to make...
by terkio
Tue Feb 19, 2019 3:33 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

I was not interested in T bias, not aware of it, actually. The preamps I know have two 10K resitors to bias the two transistor bases to ground, without a common resistor. I know this gives a poor common mode rejection, however, if my understanding is right, this is trouble only when the mic has poor...
by terkio
Tue Feb 19, 2019 2:16 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

2) "fc regardless of the gain" is a very interesting feature. I firstly presumed that was the reason to split and inject at the non inverting inputs, but I was enable to calculate or prove this feature. 3) I see such a ripple removing cap on the Samuel Groner schematic ( Monte Generoso ). I thought ...
by terkio
Tue Feb 19, 2019 12:41 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

This is a draft schematic for the latest Flat Balanced Input Moving Coil Phono Preamp using the ZTX851. https://www.proaudiodesignforum.com/images/jpg/Flat_Balanced_Input_Moving_Coil_Phono_Preamp_ZTX851_1000_BW.jpg Flat Balanced Input Moving Coil Phono Preamp using the ZTX851. Printable copy: httsp...
by terkio
Tue Feb 12, 2019 11:52 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

I suspect the Beta mismatch to make trouble of the same kind and amplitude as the Vbe mismatch. This has to do, about how much the servo will have to compensate to balance the circuit. I am using a similar design in mike preamps using two LM394 where balancing is done with a trimmer. They work perfe...
by terkio
Tue Feb 12, 2019 7:48 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

I think Hfe un balance should be considered as well as Vbe un balance. Assuming ZTX851 Hfe=150 5% accuracy, with R4=R5=499 and Ic=5.5mA: Delta Vb=499x( 5.5/150)x0.05 mV. That is about 1mV Delta Vb, to compare to a lower than 1mV Delta Vbe achieved by matching. Who knows ZTX851 Hfe tempco ? About sma...
by terkio
Thu Feb 07, 2019 8:44 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Thanks for your welcome and fast answer. I forgot to mention the preamps I worked on use two LM394 cross parallelled. I am impressed by the ZTX851 and the most excellent work you made from it. The noise figure is great. I see matching can be better than the 500uV I figured from reading earlier posts...
by terkio
Thu Feb 07, 2019 5:44 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 483
Views: 174970

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

I am very interested in this thread. I built a mic preamplifier using 4 modules based on the Grahem Cohen double balanced design. I am so found of this design, I wish to build more, but the major trouble is the availability of low noise matched bjts. I found this: AS194 AS394 Matched NPN transistor ...